The general aim of this project is the elucidation of genetic structural features of murine retroviruses which are relevant to mechanisms of viral replication, viral recombination, and viral oncogenicity. Current studies include the identification of cellular genetic sequences which undergo recombination with murine leukemia viruses in infected mice. The studies are directed at identifying cellular sequences which recombine with viruses of different oncognenic activity (e.g., lymphatic leukemia vs. erythroleukemia) as well as indentifying virally acquired cellular sequences from different mouse strains. The initial findings demonstrate that an erythroleukemia virus and a lymphatic leukemia virus selectively recombine with different sets of endogenous proviral sequences in NFS mice. These studies are currently being extended to other mouse strains and leukemia viruses. In addition to the above studies, an assay for murine retroviruses has been developed which enables one to quantitate and recover specific viruses in complex virus mixtures. The assay, which utilizes monoclonal antibodies to detect virus-encoded cell surface antigens, is designed for the isolation of endogenous viruses from mice, the isolation of viral mutants, and in vitro recombination studies.